The layers of the OSI model

 

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The layers of the OSI model

The CCNA exam might ask you to provide at least three reasons that the "industry" uses layered interconnection models. The basic reason for using a layered networking approach is that a layered model takes a task, such as data communications, and breaks it into a series of tasks, activities, or components, each of which is defined and developed independently. Examples of layered networking models include the seven-layer OSI model (which you need to know inside and out) and the Department of Defense (DOD) five-layer model (which you don't). It isn't important for you to memorize the ISO/OSI Reference Model's layers; but it's useful to know that they exist, and that each layer cannot work without the services provided by the layer below it.

Reasons for a layered model

Expect to see a question on the exam that asks you to identify the reasons a layered model is used in internetworking. Actually, a myriad of reasons exist for why a layered model is used, but you should memorize these possible responses:
 

  • Change: When changes are made to one layer, the impact on the other layers is minimized. If the model consists of a single, all-encompassing layer, any change affects the entire model.

  • Design: A layered model defines each layer separately. As long as the interconnections between layers remain constant, protocol designers can specialize in one area (layer) without worrying about how any new implementations affect other layers.

  • Learning: The layered approach reduces a very complex set of topics, activities, and actions into several smaller, interrelated groupings. This makes learning and understanding the actions of each layer and the model generally much easier.

  • Troubleshooting: The protocols, actions, and data contained in each layer of the model relate only to the purpose of that layer. This enables troubleshooting efforts to be pinpointed on the layer that carries out the suspected cause of the problem.

  • Standards: Probably the most important reason for using a layered model is that it establishes a prescribed guideline for interoperability between the various vendors developing products that perform different data communications tasks. Remember, though, that layered models, including the OSI model, provide only a guideline and framework, not a rigid standard that manufacturers can use when creating their products.
     

Under its official name, the Open Systems Interconnection Reference Model, or the OSI model, was developed by the International Organization for Standardization, which uses the abbreviation of ISO. And, yes, the full acronym of the OSI is ISO OSI.

 

The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer. In essence, the OSI model prescribes the steps to be used to transfer data over a transmission medium from one networked device to another. The OSI model is a seven-layer model developed around five specific design principles:

 

  • Whenever a discrete level of abstraction is required, a new layer should be created.

  • Each layer of the model should carry out a well-defined function.

  • The function of each layer should define internationally standardized protocols.

  • The boundaries of the layers should be placed to minimize the flow of information across interfaces.

  • There should be a sufficient number of layers defined to prevent unnecessary grouping of functions and the number of layers should also be small enough so that the model remains manageable.

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